More π, less Gap: Band Gap Tuning in Hybrid Antimony Halide Charge Transfer Semiconductors

Organic cations can directly contribute to the electronic structure in hybrid organic–inorganic (HOI) materials, resulting in the formation of charge transfer semiconductors. Systematic tuning of the band gap by varying the organic cation in HOI charge transfer semiconductors remains underexplored. Here, we report the synthesis and characterization of (phenH 2 )SbCl 5 (phenH 2 = 1,10‐phenanthrolinium) and investigate how the extended π‐system influences optical properties compared to the previously reported (2,2‐bpyH 2 )SbCl 5 (2,2‐bpyH 2 = 2,2′‐bipyridinium). Both compounds feature tetrameric [Sb 4 Cl 20 ] 8‐ units and exhibit charge–transfer band gaps. Diffuse‐reflectance spectroscopy shows a red‐shifted absorption onset for (phenH 2 )SbCl 5 relative to (2,2‐bpyH 2 )SbCl 5 . The band gap decreases from 2.6 eV in (2,2‐bpyH 2 )SbCl 5 to 2.1 eV in (phenH 2 )SbCl 5 , a reduction of >0.4 eV. Density‐functional theory (DFT) calculations indicate that this red‐shift arises from increased contribution of the phenanthroline π*‐orbitals to the conduction band. This demonstrates that systematic variation of the organic cation's π‐conjugation can provide a strategy for band gap tuning in charge–transfer HOI antimony halides.